1. Field of the Invention
The present invention is in the field of methods for preventing or reducing levels of lead in potable water sources, i.e., drinking water. It has long been known that there is a strong link between lead contamination in drinking water and adverse health effects in humans. Even at levels below the maximum contaminant level goals set by the U.S. Environmental Protection Agency set pursuant to the Safe Drinking Water Act passed by Congress in 1974, lead can cause serious damage to the brain, kidneys, nervous system and red blood cells. Proposed regulations may set maximum contaminant levels for lead as low as 20 .mu.g/L of drinking water, which is 20 part per billion (ppb).
Elevated blood-lead levels have long been linked to a wide range of deleterious health effects, particularly among young children; and severe retardation and even death at very high levels (80-100 .mu.g/dL) can result. There is a negative impact of lead on cognitive performance as measured by IQ tests and school performance which occurs at moderate-to-high blood-lead levels (30-40 .mu.g/dL). Lead has also long been associated with elevated blood pressure, hypertension, strokes and heart attacks in adults.
Lead in the drinking water supply is not from the source water per se; the majority of it results from the corrosive action of the water on the lead-containing materials from which the parts of the water distribution system are constructed. Water leaving the water treatment plant is usually relatively lead-free. However, pipes, solder, fluxes, and alloys containing lead, e.g., brass and bronze fittings, are corroded by water with the result that lead is ultimately a contaminant of the water as it issues from the consumer's tap.
Lead levels in drinking water are a complicated function of the specific quality of the water, i.e., its corrosivity, the materials comprising the distribution and household plumbing systems, pipe geometry, the length of time the water is in contact with the plumbing materials, water temperature, nature of the pipe deposit, and other factors. All of these variables help to establish the ultimate amount of lead contamination and the corresponding risk to human health.
Lead has been used in water distribution systems since ancient times, particularly because its passive oxides make it highly resistant to corrosion and attack by natural waters which it is used to carry. However, even though only minute amounts of lead dissolve in water, lead is an active and accumulative toxicant.
Different factors can accelerate the dissolution of lead in water. For example, lead is subject to corrosion in water at neutral and alkaline pH, and galvanic corrosion can occur at lead/copper joints. It is the corrosion products from galvanic attack that raise lead levels in first-drawn waters that have lain stagnant within household plumbing for a period of time. And, in general, lead concentrations in waters exposed to lead surfaces will be higher in standing water than in flowing water. Lead corrosion increases with the oxygen content of the water.
Hard waters (hardness greater than 120 mg/L CaCO.sub.3) are less corrosive to lead than soft waters. The solubility of lead as a function of pH is dependent upon the alkalinity of the drinking water. Soluble lead (Pb.sup.+2) is the dominate species only when pH and alkalinity are low. In low alkaline waters (less than 50 mg/L CaCO.sub.3), total lead concentration is highly pH sensitive. Lead solubility decreases rapidly with increasing pH at low alkalinity. With high alkalinity waters (greater than 100 mg/L CaCO.sub.3), the solubility of lead is insensitive to pH over a range of 6.5 to 8. Lead solubility increases with temperature.
The presence of orthophosphate ions in a water-carbonate system has a very large influence on lead solubility. The impact of orthophosphate ions significantly changes the broad generalization of pH and alkalinity on lead solubility, since a number of sparingly soluble lead phosphate compounds can be formed, many of which have much lower solubilities than lead carbonates. But, in high-hardness, high-alkaline waters, the addition of orthophosphate to control lead may be limited.
Polyphosphate (metaphosphate and pyrophophate) considerably inhibit lead solubility compared to control, but are less effective than orthophosphate.
2. Brief Description of the Prior Art
There are relatively few options available to minimize lead and/or control corrosion/scale in both the distribution and house plumbing systems. These include the use of lead-free solders, replacing lead lines, flushing the system prior to use, and chemical treatment. Chemical treatment can consist of pH control, alkalinity control, and the addition of specific corrosion inhibitors. The present invention is concerned with the last option, and chemical treatments of this type which have been utilized heretofore will be briefly described.
The most effective means to reduce both lead solubility and provide corrosion control to the overall distribution system is through the use of chemical treatments. There have been three basic building blocks used in potable water systems: orthophosphate, polyphosphate and silicates, all with or without zinc.
The use of orthophosphate has reduced lead solubility in both low- and high-alkalinity waters. An orthophosphate concentration of approximately 1 to 2 mg/L PO.sub.4 can be effective in reducing lead solubility over a much lower pH range than would be possible by using pH-carbonate adjustment.
Adding zinc/polyphosphate to municipal distribution systems has been an effective treatment program for controlling corrosion and scale, as well as stabilizing iron and manganese. Although polyphosphates are not as effective as orthophosphate in reducing lead solubility, the use of zinc/polyphosphate has broad applicability. The effective pH range is 6 to 7.5, but maintaining the pH above neutral is recommended.
Treatments utilizing silicates appear to have a retarding effect on lead solubility, but require a relatively long period of time, approximately 8 to 9 months, to show reductions in lead concentrations. This long-term effect can be explained by the slow formation of a kinetically-inhibited lead silicate film. Silicate treatments, however, are not recommended for control of lead solubility in distribution systems.
Further details regarding the above known chemical treatments to reduce lead solubility may be found in Boffardi, "Minimization of Lead Corrosion in Drinking Water", Materials Performance, 29(8), 45 (1990).
Moran et al., U.S. Pat. No. 4,613,450 discloses corrosion inhibitors comprising members of the fluorophosphate family, including sodium monofluorophosphate. However, these are said to be useful for protecting metallic surfaces of installations and devices using water as energetic or thermic fluid, i.e., for heating and cooling. The only metals which are mentioned are iron and its alloys, particularly galvanized steel, copper and its alloys, and aluminum and its alloys. Thus, there is no suggestion of the use of sodium monofluorophosphate to inhibit the solubility of lead in potable water sources.
Sodium monofluorophosphate is the most widely accepted dentifrice additive to reduce dental decay. In aqueous solution as well as in a paste, it has been reported to be effective in treating sensitive teeth.